The present invention relates generally to electrodeless metal halide discharge lamps and, more particularly, to a starting system therefor.
In an electrodeless metal halide lamp, an arc discharge is generated by establishing a solenoidal electric field in a gas contained within an arc tube. The solenoidal electric field is created by the time-varying magnetic field of an excitation coil which is disposed about the arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma.
In effect, the excitation coil acts as a primary coil, and the plasma functions as a single-tum secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
At room temperature, the solenoidal electric field produced by the excitation coil is typically not high enough to ionize the gaseous fill and thus initiate the arc discharge. Hence, a starting system is required. Electrodeless metal halide lamps can be started using a passive resonant circuit which provides the high voltage necessary to initiate the plasma in a starting tube. One approach is to use a parallel LC network with an adjustable capacitor for tuning the starting circuit to resonance. Presently available starting networks are manually operated and must be removed completely when the lamp starts. Otherwise, if the starting network is left in place after starting, a plasma would be maintained in the starting tube, and the lamp envelope between the starting tube and the main lamp cavity would be damaged in a short time, e.g., within a few hours of operation. Requiring that the starting network be manually removed limits the use of these lamps to applications where the network is accessible.
Accordingly, it is desirable to provide a tuned starting circuit which does not require manual operation and removal after starting.